Near-infrared reflectance spectroscopy (NIRS) analyses of forages are routinely used to predict forage quality and, consequently, animal performance. In this study, corn silage hybrids ensiled for up to 150 d (n = 74) were used to assess the relationships between NIRS NDF-related analyses conducted by 2 commercial laboratories and in situ NDF degradability variables. The in situ analysis involved 3 ruminally-cannulated cows (DMI, 26.8 ± 1.0 kg/d; milk yield, 39.8 ± 2.96 kg/d). Corn silage samples, dried and ground through a 4-mm sieve, were incubated in the rumen of the cows for up to 72 h and separately for 240 h (indigestible NDF, iNDF). A separate data set of experiments conducted at The Pennsylvania State University (n = 483 observations) was used to evaluate the relationships of undigested NDF (uNDF) of TMR or corn silage and in vivo total-tract NDF/ADF digestibility, DMI, and milk production. The relationships between in situ NDF degradability variables [b, potentially degradable NDF (mean ± SD; 95 ± 5.9%); c, rate of NDF degradation (1.14 ± 0.29%/h); ED, effective NDF degradability (36 ± 3.7%; at 2%/h passage rate); 24, 48, and 72 h NDF degradability (19.4 to 55.9, ± 5.40 to 3.68%); and iNDF (11.1 ± 1.37% of DM)] and NIRS analyses [NDF, % of DM (37 ± 2.5%); lignin, % of DM (3.3 ± 0.26%); NDF 24, 30, 48, or 240 h degradability (from 13.0 to 82.5%, ± 0.90 to 4.75%); total-tract NDF digestibility (34.1 ± 5.14%); and uNDF, % of DM or NDF (9.1 ± 1.37 and 24.5 ± 2.58%, respectively)] were non-existent or poor (R² ≤ 0.10), except between iNDF and NDF (R² ≤ 0.24; P < 0.001). The relationship of uNDF with lignin and 30-h NDF degradability was high (R² = 0.73 and 0.88, respectively; P < 0.001). The relationship of uNDF and in vivo NDF or ADF digestibility was moderate (R² = 0.29 and 0.43; P < 0.001). There was no relationship between uNDF and DMI or milk yield (P > 0.63). Overall, the relationship between NIRS fiber degradability analyses and in situ NDF degradability was poor; uNDF was moderately related to in vivo fiber digestibility, but had no relationship with DMI or milk yield in dairy cows.